[RFE / Photo] Add parametric, Δe (color similarity) and edge-aware selection / masking

[lphilpot]

It would be very useful to have two mask related capabilities, both separately and together.

• Parametric masks -- These can be adjusted to honor constraints such as hue, luminosity, etc.
   
• Δe (color similarity) masks -- Affinity Photo has selection by color but it's relatively primitive in terms of control. Being able to specify hue, luminosity and chroma parameters would enable precise selections.
   
• Edge-aware masks with adjustable edge feathering and blur -- The mask boundary would automatically follow contrast edges per the adjustment settings.

Both of these capabilities could help make complex selections / masks much easier to create and refine. Implementation examples can be found in the darktable and ART raw editors.